Scientists have unravelled the paradox of how a carcinogenic poison can also act as a treatment for cancer. The discovery could lead to better uses for arsenic in therapies for leukaemia with fewer side effects.

Arsenic is a poisonous metalloid that has long been linked to an increased risk of cancers, diabetes, thickening of the skin, diseases of the liver and digestive complications. But, since the late 1700s, compounds of the metalloid have also been used to treat leukaemia. As drugs have improved, use of the metalloid has declined, though it is still used for a rare form called acute promyelocytic leukaemia.

How the metalloid can be harmful and beneficial has mystified scientists. Ronald Hay, of the University of Dundee, led a team of researchers, including colleagues from the University of Paris, to crack the mystery by watching the behaviour of the arsenic compounds in animal cells. He found that arsenic acts like a glue, sticking itself to a type of molecule involved in leukaemia. This molecule, called Sumo, is then attacked by an enzyme called RNF4, a process that also destroys the cancer-causing proteins.

"Our discovery is key to understanding how we can enhance the anti-cancer properties of this poison," said Hay. "Knowing the specific molecules involved allows us to work on creating more targeted and effective cancer drugs with fewer side effects." The results were published yesterday in Nature Cell Biology.

Michael Tatham, also involved in the research, said: "People have been [trying] for the past 20 years to piece together how treating patients with arsenic results in the death of leukaemia cells. We have added a major piece to that puzzle."

Lesley Walker, Cancer Research UK's director of information, welcomed the research. "Discovering which molecules are involved in this process is an exciting step forward in understanding this complex paradox - how can a chemical that causes cancer also cure it?

"It's a great piece of science that will hopefully lead to the development of drugs that home in on specific cancer-causing proteins to beat the disease."

· This article was amended on Tuesday April 15 2008. Arsenic is a metalloid, rather than a metal, as we described it in an article headed Scientists solve riddle of arsenic cancer treatment. Metalloids have properties of both metals and non-metals. This has been corrected.